Selected lines of mice are being used to investigate physiological factors in the development of obesity. Emphasis will be directed toward determining how genetic predisposition and nutritional factors interact to modify the phenotypic expression of obesity. The obese strain (M16) was derived from the Institute of Cancer Research albino strain (ICR) by selection for increased growth rate. A second line selected for rapid growth rate (H6) shows no tendency toward obesity. Major factors to be considered in this comparative approach will be genetic effects, nutritional state, age and sex. Nutrition regimes will include ad libitum and controlled feed intake. Adipose cellularity of epididymal, inguinal and retroperitoneal fat pads will be determined. Rates of synthesis and turnover of triacylglycerols in specific tissues will be estimated by in vivo and in vitro techniques. Several enzymes involved in lipid metabolism will be assayed in liver and adipose tissue.